Transistors can be divided into two main types: bipolar junction transistors and field-effect transistors. Both types share a common structure comprising three electrodes with a semiconductive material disposed therebetween in a channel region. The three electrodes of a bipolar junction transistor are known as the emitter, collector and base, whereas in a field-effect transistor the three electrodes are known as the source, drain and gate. Bipolar junction transistors may be described as current-operated devices as the current between the emitter and collector is controlled by the current flowing between the base and emitter. In contrast, field-effect transistors may be described as voltage-operated devices as the current flowing between source and drain is controlled by the voltage between the gate and the source.
Transistors can also be classified as p-type and n-type according to whether they comprise semiconductive material which conducts positive charge carriers (holes) or negative charge carriers (electrons) respectively. The semiconductive material may be selected according to its ability to accept, conduct, and donate charge. The ability of the semiconductive material to accept, conduct, and donate holes or electrons can be enhanced by doping the material. The material used for the source and drain electrodes can also be selected according to its ability to accept and inject holes or electrons. For example, a p-type transistor device can be formed by selecting a semiconductive material which is efficient at accepting, conducting, and donating holes, and selecting a material for the source and drain electrodes which is efficient at injecting and accepting holes from the semiconductive material. Good energy-level matching of the Fermi-level in the electrodes with the HOMO (Highest Occupied Molecular Orbital) level of the semiconductive material can enhance hole injection and acceptance. In contrast, an n-type transistor device can be formed by selecting a semiconductive material which is efficient at accepting, conducting, and donating electrons, and selecting a material for the source and drain electrodes which is efficient at injecting electrons into, and accepting electrons from, the semiconductive material. Good energy-level matching of the Fermi-level in the electrodes with the LUMO (Lowest Unoccupied Molecular Orbital) level of the semiconductive material can enhance electron injection and acceptance.
Transistors can be formed by depositing the components in thin films to form thin film transistors. When an organic material is used as the semiconductive material in such a device, it is known as an organic thin film transistor.
Various arrangements for organic thin film transistors are known. One such device is an insulated gate field-effect transistor which comprises source and drain electrodes with a semiconductive material disposed therebetween in a channel region, a gate electrode disposed over the semiconductive material and a layer of insulting material disposed between the gate electrode and the semiconductive material in the channel region.
An example of such an organic thin film transistor is shown in FIG. 1. The illustrated structure may be deposited on a substrate (not shown) and comprises source and drain electrodes 2, 4 which are spaced apart with a channel region 6 located therebetween. An organic semiconductor 8 is deposited in the channel region 6 and may extend over at least a portion of the source and drain electrodes 2, 4. An insulating layer 10 of dielectric material is deposited over the organic semi-conductor 8 and may extend over at least a portion of the source and drain electrodes 2, 4. Finally, a gate electrode 12 is deposited over the insulating layer 10. The gate electrode 12 is located over the channel region 6 and may extend over at least a portion of the source and drain electrodes 2, 4.
The structure described above is known as a top-gate organic thin film transistor as the gate is located on a top side of the device. Alternatively, it is also known to provide the gate on a bottom side of the device to form a so-called bottom-gate organic thin film transistor.
An example of such a bottom-gate organic thin film transistor is shown in FIG. 2. In order to show more clearly the relationship between the structures illustrated in FIGS. 1 and 2, like reference numerals have been used for corresponding parts. The bottom-gate structure illustrated in FIG. 2 comprises a gate electrode 12 deposited on a substrate 1 with an insulating layer 10 of dielectric material deposited thereover. Source and drain electrodes 2, 4 are deposited over the insulating layer 10 of dielectric material. The source and drain electrodes 2, 4 are spaced apart with a channel region 6 located therebetween over the gate electrode. An organic semiconductor 8 is deposited in the channel region 6 and may extend over at least a portion of the source and drain electrodes 2, 4.
The conductivity of the channel can be altered by the application of a voltage at the gate. In this way the transistor can be switched on and off using an applied gate voltage. The drain current that is achievable for a given voltage is dependent on the mobility of the charge carriers in the organic semiconductor in the active region of the device (the channel region between the source and drain electrodes). Thus, in order to achieve high drain currents with low operational voltages, organic thin film transistors must have an organic semiconductor which has highly mobile charge carriers in the channel region.
There are various compound types that have been developed in recent years that are potentially suitable for use as the semiconductive material in organic thin film transistors. One such class of particular importance is the so-called small molecule semiconductors. These are non-polymeric semiconducting organic molecules. Typical examples include pentacene derivatives and thiophene derivatives.
Although small molecule semiconductor materials can exhibit high mobilities due to their highly crystalline texture (particularly as thermally evaporated thin films) it can often be difficult to obtain repeatable results from solution processed films due to their poor film forming properties. Issues with material reticulation from and adhesion to substrates, film roughness and film thickness variations can limit the performance of these materials in devices. Film roughness can be a further problem for top-gate organic thin film transistor devices, as the accumulation layer is formed at the uppermost surface of the semiconductor layer.
To overcome this problem, the use of semiconductor blends consisting of small molecules and polymers has been developed. The motivation for using such blends is primarily to overcome the poor film forming properties of the small molecule semiconductor materials. Blends of small molecules with polymers exhibit superior film forming properties to the small molecule component due to the excellent film forming properties of polymer materials.
A few examples of such blends (semiconductor-semiconductor or semiconductor—insulator) in the literature include Smith et. al., Applied Physics Letters, Vol 93, 253301 (2008); Russell et. al., Applied Physics Letters, Vol 87, 222109 (2005); Ohe et. al., Applied Physics Letters, Vol 93, 053303 (2008); Madec et. al., Journal of Surface Science & Nanotechnology, Vol 7, 455-458 (2009); and Kang et. al., J. Am. Chem. Soc., Vol 130, 12273-75 (2008). Another example is WO 2005/055248, which discloses the preparation of organic thin film transistors in which the semiconductor layer is a blend of a pentacene derivative with a semiconductor binder such as a poly(triarylamine) or poly(9-vinylcarbazole) deposited from a solution thereof in a solvent. The solvent used in one of the examples is anisole. However, there is no disclosure or suggestion in WO 2005/055248 that its use would reduce contact resistance.
Chung et al, Thin Solids Films, published online Mar. 6, 2010, discloses the preparation of organic thin film transistors in which the semiconductor layer is a blend of TIPS-pentacene with a poly(triarylamine) deposited from a solution thereof in a solvent. The solvent used in one of the examples is anisole. The authors discuss some properties resulting from the use of different solvents to deposit the blend, such as enhanced morphology and charge mobility in the channel region. However, there is no correlation between the disclosed properties and contact resistance.
The prior art relating to semiconductor small molecule-polymer blends has focused on improving the charge mobility and on good stability. Selection of the semiconductor material(s) and their ratios in the blend in order to optimise the field effect mobility has been the major area of concern. No research has been conducted into the solvent selection to reduce contact resistance in devices incorporating semiconductor blends.
The contact resistance in organic thin film transistors is a crucial parameter to minimise (ideally eliminate), particularly for short channel length devices (<20 μm), where this resistance can contribute a significant proportion to the total channel resistance in the device. The higher the contact resistance in the device, the higher the proportion of the applied voltage is dropped across this and, as a result, the lower the bias across the channel region is achieved. A high contact resistance has the effect of a much lower current level being extracted from the device due to the lower bias applied across the channel region, and hence lower device mobility.
Minimising the contact resistance is particularly important in devices incorporating high mobility semiconductor materials, as the channel resistance is lower (high conductivity) than that observed in for example amorphous phase polymer semiconductors alone at the same channel length. This is therefore of particular importance for semiconductor materials comprising crystalline materials that exhibit high mobilities. For small molecule materials such as benzothiophene derivatives and pentacene derivatives a low solubility is found (0.4% w/v limit) and a poor film quality is also found. Above this concentration, the material will form a crystalline precipitate in the host solvent. In this case adding a polymer semiconductor becomes important from the point of view of increasing the solution viscosity to improve film formation and to act as a binder to ensure a continuous film is obtained spanning from the source to drain electrodes. The polymer semiconductor exhibits a high solubility in the same solvents (above 2% w/v).
Conventionally, the contact resistance in organic thin film transistors is reduced solely by applying surface treatment layers to the source and drain electrodes prior to depositing the semiconductor film or by changing the metal to a higher work function metal as necessary to inject charges to the HOMO level (for a p-type material). Such treatment layers (typically self assembled monolayers applied from solution or vapour phase) are used to produce a dipole layer at the metal surface to effectively shift the work function of the source and drain contacts to align with the HOMO level in the semiconductor and therefore reduce the barrier for charge injection from metal to the semiconductor.
In some instances, these methods alone are not sufficient to minimise the contact resistance. There is a need to find additional ways of reducing the contact resistance of organic thin film transistors comprising semiconductor blend layers.